Cellular differentiation of multicellular organisms is controlled by hormones and polypeptide growth factors. These diffusible molecules allow cells to communicate with each other and act in concert to form tissues and organs, and to repair and regenerate damaged tissue. Examples of hormones and growth factors include the steroid hormones, parathyroid hormone, follicle stimulating hormone, the interferons, the interleukins, platelet derived growth factor, epidermal growth factor, and granulocyte-macrophage colony stimulating factor, among others.
Hormones and growth factors influence cellular metabolism by binding to receptor proteins. Certain receptors are integral membrane proteins that bind with the hormone or growth factor outside the cell, and that are linked to signaling pathways within the cell, such as second messenger systems. Other classes of receptors are soluble intracellular molecules.
Of particular interest, from a therapeutic standpoint, are the interferons (reviews on interferons are provided by De Maeyer and De Maeyer-Guignard, "Interferons," in The Cytokine Handbook 3.sup.rd Edition, Thompson (ed.), pages 491-516 (Academic Press Ltd. 1998), and by Walsh, Biopharmaceuticals: Biochemistry and Biotechnology, pages 158-188 (John Wiley & Sons 1998)). Interferons exhibit a variety of biological activities, and are useful for the treatment of certain autoimmune diseases, particular cancers, and the enhancement of the immune response against infectious agents, including viruses, bacteria, fungi, and protozoa. To date, six forms of interferon have been identified, which have been classified into two major groups. The so-called "type I" interferons include interferon-.alpha., interferon-.beta., interferon-.omega., interferon-.delta., and interferon-.tau.. Currently, interferon-.gamma. and one subclass of interferon-.alpha. are the only type II interferons.
Type I interferons are thought to be derived from the same ancestral gene, and the type I interferons have retained sufficient similar structure to act by the same cell surface receptor. The .alpha.-chain of the human interferon-.alpha./.beta., receptor comprises an extracellular N-terminal domain, which has the characteristics of a class II cytokine receptor. Interferon-.gamma. does not share significant homology with the type I interferons or with the type II interferon-.alpha. subtype, but shares a number of biological activities with the type I interferons.
In humans, at least 16 non-allelic genes code for different subtypes of interferon-.alpha., while interferons .beta. and .omega. are encoded by single genes. Type I interferon genes are clustered in the short arm of chromosome 9. Unlike typical structural human genes, interferon-.alpha., interferon-.beta., and interferon-.omega. lack introns. A single gene for human interferon-.gamma. is localized on chromosome 12 and contains three introns. To date, interferon-.tau. has been described only in cattle and sheep, while interferon-.delta. has been described only in pigs.
It is believed that all cells may be capable of producing interferons .alpha. and .beta. in response to viral infection, double-stranded RNA molecules, growth factors, and cytokines. Normally, however, interferon-.alpha. is produced by lymphocytes, macrophages, and monocytes, while interferon-.beta. is synthesized by fibroblasts and some epithelial cells. Interferon-.gamma. is produced by T cells or natural killer cells.
Clinicians are taking advantage of the multiple activities of interferons by using the proteins to treat a wide range of conditions. For example, one form of interferon-.alpha. has been approved for use in more than 50 countries for the treatment of medical conditions such as hairy cell leukemia, renal cell carcinoma, basal cell carcinoma, malignant melanoma, AIDS-related Kaposi's sarcoma, multiple myeloma, chronic myelogenous leukemia, non-Hodgkin's lymphoma, laryngeal papillomatosis, mycosis fungoides, condyloma acuminata, chronic hepatitis B, hepatitis C, chronic hepatitis D, and chronic non-A, non-B/C hepatitis. The U.S. Food and Drug Administration has approved the use of interferon-.beta. to treat multiple sclerosis, a chronic disease of the nervous system. Interferon-.gamma. is used to treat chronic granulomatous diseases, in which the interferon enhances the patient's immune response to destroy infectious bacterial, fungal, and protozoal pathogens. Clinical studies also indicate that interferon-.gamma. may be useful in the treatment of AIDS, leishmaniasis, and lepromatous leprosy.
Although new uses of known interferons may be discovered, a need exists for the provision of new interferons for biopharmaceuticals.